This application relates to an apparatus for controlling fluid flow to two different hydraulic circuits. More particularly, the application relates to a priority valve for controlling fluid flow to a primary hydraulic steering circuit and an auxiliary hydraulic circuit in a hydrostatic load sense vehicle steering system.
A hydrostatic load sense system for vehicle steering often includes a priority valve for controlling fluid flow from a pump to a primary steering circuit and an auxiliary hydraulic circuit. The pump may have either a fixed or a variable displacement. The steering circuit typically includes a steering motor and a steering controller that responds to movement of a steering wheel to direct fluid flow to the steering motor. The auxiliary circuit includes other hydraulically operated devices, such as brakes or an implement like a backhoe. When the vehicle's operator is not rotating the steering wheel, the priority valve directs most of the fluid from the pump to the auxiliary circuit where it is either bypassed to a reservoir, or utilized to operate the auxiliary devices. A small amount of fluid goes to the steering circuit to maintain a standby fluid pressure in the steering circuit and replenish any fluid lost through leakage.
When the steering wheel is rotated to steer the vehicle, the steering controller directs fluid flow to the steering motor and signals the priority valve to direct fluid flow from the pump to the steering circuit, in preference to the auxiliary circuit. The precise division of fluid flow between the steering circuit and the auxiliary circuit is a function of operator demand and the load on the steering motor. For example, if the vehicle operator turns the steering wheel rapidly and through a large angle of rotation, the signal from the controllor indicates that the steering circuit has a high demand for fluid flow and pressure. Whenever the steering circuit has a high demand for fluid flow and pressure, the priority valve shifts to a priority position in which all fluid flow from the pump is directed to the steering circuit if the pump flow is insufficient to satisfy the demand. If the steering wheel is turned less rapidly and through a smaller angle of rotation, the signal from the controller indicates less of a demand for fluid flow and pressure. When the fluid flow and pressure being directed to steering are sufficient to satisfy that demand, the priority valve can shift from its priority position and begin to direct fluid flow to the auxiliary circuit. When movement of the steering wheel stops and/or the demand for fluid flow and pressure in the steering circuit has been satisfied, the priority valve can shift further toward an auxiliary position in which fluid is supplied primarily to the auxiliary circuit.
A specific load sense hydrostatic steering system with a priority valve is described and illustrated in U.S. patent application Ser. No. 243,497 which is assigned to the assignee of the present invention. In that system, a priority valve bypasses flow to the auxiliary circuit when there is no steering. In response to an operator's initiation of steering, a control pressure signal is produced which anticipates a steering demand, and causes rapid movement of the priority valve to its priority position. During the steering manuever, the fluid flow to the steering motor is directed through a main flow control orifice in the controller. The main flow control orifice varies in area according to operator demand and steering load. The flow through that orifice is used to vary the control pressure signal, and thereby control the position of the priority valve, in accordance with operator demand and steering load.
In the system of U.S. application Ser. No. 243,297 the priority valve has a valve spool which is controlled by fluid pressures in a pilot flow which branches from the priority port of the priority valve. The pilot flow is directed through a fixed size orifice in the priority valve, and through the steering controller to a reservoir when there is no steering. Fluid pressure at the priority port is communicated with one end of the valve spool. Fluid pressure at the downstream side of the fixed orifice in the pilot flow is communicated with the other end of the priority valve spool. The pressures are applied to equal surface areas at the ends of the valve spool. The difference between the pressures applied to the valve spool produces a net force tending to move the valve spool in one direction. The net pressure force is opposed by the force of a biasing spring that acts on the valve spool. The biasing spring acts in a direction so as to bias the valve spool towards its priority position. The biasing force of the spring can be overcome by a sufficiently large net pressure force resulting from a large pressure differential across the fixed orifice which is applied to the opposite ends of the valve spool.
The steering system of application Ser. No. 243,497 responds to initiation of steering to provide a pressure signal which anticipates demand for fluid flow and pressure by the steering circuit. Specifically, upon initiation of a steering maneuver, and just before or as the main flow control orifice in the controller begins to direct fluid flow for steering, the system restricts the pilot flow, and abruptly instantaneously alters the pressure differential applied to the spool of the priority valve. A transient pressure increase is applied to the valve spool and, along with the biasing spring, applies a high biasing force to the valve spool urging the valve spool toward its priority position. The fluid delivered by the pump will ordinarily not be sufficient to balance the high biasing force caused by the transient pressure increase. Thus, the valve spool moves quickly toward its priority position before or as the steering controller is demanding fluid flow and pressure. In a system utilizing the principles of application Ser. No. 243,497, increased fluid flow and pressure to the steering circuit should be contemporaneous with or anticipate the demands of the steering circuit.
During steering, when the main flow orifice in the controller begins to direct fluid flow for steering, the pressure at the priority port of the priority valve continues to act on the one end of the priority valve spool, and the pressure at the downstream side of the main flow control orifice is connected to the other end of the valve spool. The transient pressure bias is dissipated by the priority valve's initial movement to its priority position. Thus, during steering, as the main flow control orifice in the controller varies in flow area in accordance with operator demand and steering load, the pressure differential applied to the priority valve spool varies. When the pressure differential reaches a predetermined point (indicating that the steering demand is being satisfied), the valve spool can shift away from its priority position and toward its auxiliary position to direct flow to the auxiliary circuit. The pressure differential is set at a predetermined high enough level so that when it is reached, the system can satisfy a steering demand while accounting for line losses which will exist in the fluid flow between the priority valve and the steering controller.
The design of the priority valve of U.S. application Ser. No. 243,497 will also cause it to try to hold a predetermined minimum pressure at the priority port when there is no steering. The pressure differential applied to the valve spool shifts the valve spool away from its priority position and toward its auxiliary position when the predetermined pressure is reached at the priority port. The predetermined minimum pressure is directly related to the pressure differential needed to satisfy a steering demand and overcome line losses. As the valve spool shifts away from its priority position, and begins to direct flow to the auxiliary circuit, the pressure at the priority port can vary with variations in the condition of the auxiliary circuit. Specifically, when the auxiliary implement is operated, there is a back pressure at the auxiliary port of the priority valve. The back pressure increases the pressure at which fluid must be delivered from the pump to overcome the back pressure. The pressure at the priority port will then exceed the predetermined minimum pressure required at the priority port. On the other hand, when the auxiliary circuit is not operating, but is bypassing all of its fluid flow to the reservoir, there is little or no back pressure at the auxiliary port of the priority valve. The pressure at the priority port is then held at the predetermined pressure. When there is no steering and little or no back pressure at the auxiliary port, the predetermined minimum pressure at the priority port could theoretically be reduced. Specifically, because the predetermined pressure in the system of application Ser. No. 243,497 is related to the pressure differential which must be applied to the priority valve spool to satisfy a steering demand and overcome associated line losses, the predetermined pressure is more than enough to overcome the line losses associated with the small pilot flow of fluid that occurs when there is no steering demand for fluid flow and pressure. The predetermined pressure could thus be reduced to a level which produces just enough pilot fluid to insure the transient pressure increase which urges the priority valve rapidly toward its priority position. In turn, the fluid flow and pressure which is required from the pump to hold the predetermined pressure at the priority port could also be reduced. If pump pressure could be reduced, there would be an energy savings, because the power for operating the pump could be reduced.